The overall objectives of the proposed research are to investigate the biochemistry, molecular pathology, and potential therapy of acute intermittent porphyria (AIP), an autosomal dominant hepatic porphyria due to the half-normal activity of hydroxymethylbilane synthase (HMBS), and congenital erythropoietic porphyria (CEP), an autosomal recessive disorder due to the markedly deficient, but not absent, activity of uroporphyrinogen III synthase (UROS). Three specific aims are proposed: 1) NMR studies will characterize the structure and reaction mechanism of human UROS and its interaction in the UROS/HMBS cytosolic complex. Possible interaction of the UROS/HMBS complex with 5-aminolevulate dehydratase (ALAD) and uroporphyrinogen decarboxylase (UROD) in a multi-enzyme complex or "metabolon" will be investigated. The subcellular location of the UROS/HMBS complex will be determined with fluorescent anti-enzyme antibodies. 2) For AIP, efforts will determine if the life-threatening, acute neurologic attacks can be prevented by liver-targeted gene therapy. Based on the evaluation of various liver-specific promoter/enhancer combinations, two optimal promoter/enhancer constructs containing the HMBS cDNA (with/without the strong alpha-galactosid nase A leader sequence) will be made and evaluated for hepatic expression and secretion following hydrodynamic delivery. The optimally expressing vector(s) with envelope serotypes (1, 5, and 8) will be injected into the portal vein of the "AIP mice" and their ability to prevent phenobarbital-induced acute porphyric attacks will be monitored by plasma and urinary ALA and porphobilinogen (PBG) levels. 3) For CEP, a viable UROS knock-in mouse model(s) will be generated using four murine UROS missense mutations expressing 0.1-10% of wild-type activity. Transfected ES cells clones are being screened and positive clones will be hyper-selected to homozygosity to assess their viability and residual UROS activity. Positive heterozygous clones will be used to generate founder mice for each muation, which will be bred to homozygosity, to each other, and to UROS heterozygous null mice, potentially generating knock-in mice with 0.05-10% of wild-type activities. These mice will be characterized biochemically, pathologically, and clinically, especially their hematologic and dermatologic manifestations. Viable CEP mice should permit studies of the disease pathophysiology and future therapeutic endeavors.